This invention relates generally to methods of making striped screens for television picture tubes, and more particularly, to an improved process for forming such screens by electrophoretic deposition.
The standard process of depositing patterns of color phosphors on the inner surface of a color picture tube faceplate is well known in the art. Essentially, the process consists of a series of several steps comprising the deposition of three different color phosphor compounds, one for each primary color. A shadow mask which is, in effect, a perforated "optical stencil," is used in conjunction with a light source to expose and screen three discrete photo-resist patterns on the faceplate. The shadow mask is "mated" to each faceplate; that is, the same mask is used in the production of a specific tube throughout the production process, and is finally installed permanently in that tube. Four engagements and four disengagements of the mask, and six exposures, are required in the standard process. Although this process is widely used, it has many disadvantages. The most significant of these lies in the many steps required in the production process including the numerous removals and installations of the mask which must remain in precise registery through the process. Another disadvantage lies in the multiple exposures required. In addition, there is a considerable waste of chemicals including the costly phosphor compounds. The reject rate in manufacture may be substantial, and is usually attributable to a breakdown at some point in the relatively complex production process.
In certain faceplate screening processes, a "master" may be used for exposing the photo-resist patterns in lieu of the mated shadow mask heretofore described. Such a master is not permanently associated with a single specific faceplate, but comprises an interchangeable component that can be used to screen production quantities of faceplates. A master, however, is used mainly in tube types that do not utilize a shadow mask, such as the beam-index tube. For shadow-mask type-tubes, interchangeable mask systems have not proved feasible in production.
An alternate means for deposition of screening compounds is that of electrophoretic deposition, also called "cataphoretic deposition." In this process, screening compounds which may include phosphor particles suspended in an electrically low conductive fluid, or "particle suspensions," are caused to migrate through an electric field in the fluid to a surface having an opposite charge, and on which the desired pattern is to be deposited. There are many advantages to the electrophoretic deposition process that makes it attractive; however, experience has revealed inherent problems that has made the process unacceptable to data as a viable alternative to the aforedescribed standard process.
U.S. Pat. No. 3,681,223 to Gupton discloses means whereby color phosphors are electrophoretically deposited on a cathode ray tube faceplate to provide a dot screen. A plurality of conductive dot patterns are deposited on the faceplate, one pattern for each color to be deposited. Each pattern includes lands of material on which the dots are to be deposited with conductive paths interconnecting the lands. The conductive patterns are deposited with a single masking operation using an interchangeable master. The different color phosphors are then successively deposited on dot patterns by electrophoresis, one color phosphor being deposited on each discrete conductive pattern. The conductive paths are said to be one-mil wide. The conductive paths for the blue and green phosphor patterns are shown as being relatively short (perhaps only as long as the heighth dimension of the tube), and are shown as being interdigitated. The conductive path for the red phosphor, however, which proceeds as a continuous one-mil strip between the interdigitations of the blue and green conductive paths is relatively long; e.g., about 900 feet long in a 25-inch picture tube. The problem of laying down a conductive path of such length with an inter-dot width of only one mil without an electrically interrupting gap is considered to be insurmountable. Even if there were no gaps, the end-to-end variation in electrical potential on the one mil strip due to the IR drop along the strip would result in uneven phosphor deposit. The process as disclosed by Gupton requires a master exposure system; as noted, interchangeable mask systems have not been commercially proven.
In Canadian Pat. No. 964,713, Standaart discloses a process for the electrophoretic deposition of color phosphors in a dot-screen pattern. A separate dot pattern with an inter-connecting conductive path is provided for each color. An electrical potential is applied to an end of each path by means of a clamping device for the deposition of each color. The means for applying the potential are not disclosed. This system is based on the use of an interchangeable mask.